Hysteresis properties are not only a critical factor in the selection of steels for use in electrical machines but, by their nature, can also provide significant information on the structural condition and/or magnetic anisotropy of ferromagnetic materials generally. With electrical steels, the energy loss itself is the usual parameter of prime interest, whereas one or both of its typically key components namely, coercivity and the remanent induction provide the sought for information.
Conventional measurements of hysteretic properties usually employ ring or strip samples, the latter being necessary when anisotropy is being explored (see, M. Emura, et al., “Angular Dependence of Magnetic Properties of 2% Silicon Electrical Steel, J. Magn. Magn. Mat., vol. 226-230, Part 2, pp. 1524-1526 (2001)). Non destructive determinations of local properties, for example to explore the relative structural damage associated with different processes used to cut electrical steel sheets, typically require either specially prepared samples (see, E. G. Araujo, et al., “Dimensional Effects on Magnetic Properties of Fe—Si Steels Due to Laser and Mechanical Cutting”, IEEE Trans. Magn., vol. 46, no. 2, pp. 213-216 (February 2010)), sophisticated techniques such as the needle probe method (see, G. Crevecoeur, et al., “Analysis of the Local Material Degradation Near Cutting Edges of Electrical Steel Sheets”, IEEE Trans. Magn., vol. 44, no. 11, pp. 3173-3176 (November 2008)) or specialized apparatus such as with the drag force method (see, I. J. Garshelis, et al., “Application of the Drag Force Method to Evaluate Magnetic Property Degradation Near the Cut Edges of Electrical Steels”, J. Appl. Phys. 109, 07E518 (2011)). All methods except drag force employ magnetic fields derived from electric currents.
Evaluating magnetic hysteresis properties is typically achieved through the use of, e.g., an Epstein frame or a single sheet tester. Measurement in such apparatus gives the global magnetic hysteresis properties averaged over the cross section and length of the sample under test. This is usually suitable for materials whose end use is in electrical machines. For many applications, however, inhomogenity of magnetic properties is the characteristic of principal interest, since variation in local properties is reflective of variation in their origins, namely: composition, microstructure, residual stress, and the detection of inhomogeneous metallurgical conditions is the underlying purpose of the measurement.
Therefore, a new magnetostatic sensor and method for obtaining comparative measurements of hysteresis components in ferromagnetic sheet materials is needed where the measurements obtained provide quantitative information which correlates directly with all of the significant aspects of conventional hysteresis loops.